1. Field of the Invention
The invention relates to processes and systems for purifying boron trichloride. In particular, the invention relates to processes and systems or apparatus which remove several critical impurities of boron trichloride to produce a highly purified final product required for some of its more stringent applications.
2. Related Art
Boron trichloride (also referred to herein as "BCl.sub.3 ") is a highly reactive compound packaged as a liquid under its own vapor pressure of 1.3 bar (130 kPa) absolute at 21.degree. C. that has numerous diverse applications. It is used predominantly as a source of boron in a variety of manufacturing processes. For example, in the manufacturing of structural materials, boron trichloride is the precursor for chemical vapor deposition ("CVD") of boron filaments used to reinforce high performance composite materials. BCl.sub.3 is also used as a CVD precursor in the boron doping of optical fibers, scratch resistant coatings, and semiconductors. Some of the non-CVD applications of BCl.sub.3 are reactive ion etching of semiconductor integrated circuits and refining of metal alloys. In metallurgical applications, it is used to remove oxides, carbides, and nitrides from molten metals. In particular, BCl.sub.3 is used to refine aluminum and its alloys to improve tensile strength.
Two of the most stringent applications for high purity BCL.sub.3 involve semiconductor and optical fiber manufacturing. In these industries the specified impurity levels in BCl3 must be of the order of 1 ppm or less in order to maintain product quality. In fact, the impurities in most commercially available BCl.sub.3 are often present at levels over two orders of magnitude beyond acceptable levels for these processes such as, for example, air, CO.sub.2, HCl, Cl.sub.2, and COCl.sub.2 ("phosgene"). Furthermore, in these particular applications, any oxygen or oxygen containing impurities (such as phosgene) in the BCl.sub.3 are especially detrimental to the manufacturing process due to the formation of certain oxide compounds. Another class of detrimental impurities in BCl.sub.3 for these processes are metal containing impurities.
Geographically, BCl.sub.3 is produced almost entirely in the United States. As of 1995, as much as 220 metric tons has been consumed in the United States where about 30% has gone into the production of boron reinforcement filaments, the remaining split primarily among semiconductor etching, Friedel-Crafts catalysis reactions, and intermediate use in pharmaceuticals. In comparison, Japan consumes 70 metric tons which was all imported from the United States. In Japan, BCl.sub.3 is used primarily in semiconductor etching and manufacture of crucibles for silicon ingots. Western European countries consumed only about 5 metric tons. (Chemical Economics Handbook, October, 1996.)
The source cost of BCl.sub.3 varies considerably per pound depending upon purity grade and supplier. There is a strong incentive to purchase BCl.sub.3 domestically at a low cost and purify the material to stringent semiconductor purity requirements of technically 1 ppm or less for the light impurities.
After extensively searching the literature and patents, there appears to be no production process technology to have been described or patented regarding how to efficiently remove various impurities from boron trichloride by an integrated purification process technology comprising several different functional chemical processes which are connected sequentially and various impurities associated with boron trichloride are removed sequentially and continuously.
The removal of some impuritites in BCl.sub.3 has been disclosed previously. In particular, most publications have focused on how to remove phosgene from boron trichloride. This is because phosgene has similar vapor pressure to BCl.sub.3 and hence becomes difficult to remove by simple distillation. The previous methods for phosgene removal from BCl.sub.3 include electrical discharge, laser pyrolysis, fractional distillation, UV photolysis, and redox chemistry.
Although the individual methods aforementioned had indicated to be able to reduce phosgene content in boron trichloride to a certain degree, these methods do have their drawbacks. For instance, the use of electrical discharge and laser pyrolysis is difficult to implement on a larger industrial scale without extensive equipment and capital costs, and therefore, the economics are not feasible. UV photolysis lacks effectiveness for phosgene removal to very low ppm levels. Further, the similarity of physical properties of phosgene and boron trichloride makes phase separation by distillation and differential surface adsorption difficult to implement in a practical manner. It is also known to use selective chemistry to remove phosgene from BCl.sub.3. In these methods phosgene in the BCl.sub.3 is allowed to oxidize molten metals such as mercury, copper, and titanium to form the corresponding metal chlorides and carbon monoxide. Although effective in removing phosgene, this approach presents problems with metal contamination, which is particularly difficult due to the volatility of metal chlorides.
In view of all the drawbacks aforementioned, the preferred process of removing phosgene is by thermal decomposition via a catalyst with a specified elevated temperature. For example, the phosgene decomposition on a preferably metal free carbonaceous catalyst was described by two earlier publications. However, in each of these two cases, other troublesome impurities were generated (chlorine in one case, and hydrogen chloride in the other) which require independent purification steps.
Another problem with known BCl.sub.3 purification methods is the need to resort to vacuum generating devices or thermal heating of source material and associated handling systems to improve the rate of vapor transport through packed beds of adsorbents or catalytic materials. In known BCl.sub.3 purification methods using packed beds such as the case of carbonaceous catalysts, there are significant pressure drops associated with packed beds when high volumetric flow rates are employed and good surface contact required. For many gases, this is not a problem. But, when it comes to BCl.sub.3, material transport through such pressure drops becomes significantly hindered due to the BCl.sub.3 liquid having only a 1.3 bar vapor pressure at ambient temperature. Thus, maintaining reasonable flow rates through such devices requires some auxiliary means of promoting flow. Conventionally, flow throughput can be advanced by either increasing upstream pressure or decreasing downstream pressure. Increasing upstream pressure can be done using commonly known techniques of gravimetric feeding, mechanical pumping, or thermal heating of source material. However, in the specific case of producing high purity corrosive gases like BCl.sub.3, the reactive nature of BCl.sub.3 makes the mechanical devices undesirable requiring high maintenance and excessive costs while providing low reliability and the increased likelihood of contamination of the BCl.sub.3 by metallic impurities. Gravimetric feeding (in other words, elevating source material relative to the rest of the system) effectively promotes flow as only 2 meter height provides almost 1 bar additional upstream pressure. However, this approach still suffers from the intolerable feature of requiring material transport through the system as entirely liquid phase instead of vapor phase. As a consequence of liquid phase present in the system, excessive contamination of BCl.sub.3 by metallic impurities can occur from enhanced liquid phase corrosion mechanisms thereby degrading product purity with detrimental metallic impurities.
One known method of increasing upstream pressure with vapor condensation downstream is to heat the source material and all associated gas handling components to an isothermal temperature. The method is feasible but requires careful temperature control to assure uniform temperature throughout the system. Although feasible, this technique becomes difficult to implement in practice especially for high capacity industrial production.
Resorting to decreasing downstream pressure has its difficulties also. The simplest approach of mechanical pumping suffers from the same problems as in the upstream case. The use of simple low temperature condensation of BCl.sub.3 downstream prevents the problems of mechanical pumping but will lead to accumulation of metallic impurities in the final product collected hence degrading purity.